1. Field of the Invention
The present invention relates to a two-stroke engine.
2. Description of the Related Art
In a known four-stroke engine which a depression is formed on the top face of the piston, the entire fuel is injected from the fuel injector toward the depression at one time, a swirl motion, swirling about the axis of the cylinder, is created in the combustion chamber, and an ignitable air-fuel mixture is formed around the spark plug due to the swirl motion (see Japanese Unexamined Utility Model application No. 1-124042).
In a two-stroke engine, however, if the entire fuel is injected from the fuel injector toward the depression, as in the above-mentioned four-stroke engine, a problem arises particularly when the engine is operating under an intermediate load.
Namely, in a two-stroke engine, a large amount of burned gas remains in the combustion chamber, and in this case, since the amount of fuel injected by the fuel injector is increased as the engine load Q/N (the amount of air Q fed into the engine cylinder/the engine speed N) becomes higher, the temperature of the combustion gas also increases as the engine load becomes higher. Accordingly, as illustrated by the curved line T in FIG. 19, the temperature of burned gas remaining in the combustion chamber increases as the engine load Q/N becomes higher. Conversely, since the amount of air fed into the engine cylinder is increased as the engine load Q/N becomes higher, the amount of burned gas S remaining in the combustion chamber is reduced as the engine load Q/N becomes higher, as illustrated in FIG. 19. In this case, the thermal energy of the entire burned gas remaining in the combustion chamber is determined by the temperature and the amount of burned gas remaining in the combustion chamber, and consequently, as illustrated by the broken line E in FIG. 19, the thermal energy of the entire burned gas remaining in the combustion chamber reaches a highest level when the engine is operating under an intermediate load, and thus the vaporization of fuel injected by the fuel injector is greatest when the engine is operating under an intermediate load.
Therefore, in a two-stroke engine, if a large amount of fuel is injected by the fuel injector at one time, when the engine is operating under an intermediate load, since the injected fuel is rapidly vaporized, an air-fuel mixture having an approximately stoichiometric air-fuel ratio, which most easily causes self-ignition, is formed before the ignition by the spark plug is carried out, and as a result, a problem occurs in that self-ignition occurs, and thus a combustion noise or knocking occurs.
To avoid this problem, if the injection time is retarded the mixing time of the injected fuel and air becomes short, and therefore, a part of the injected fuel sufficiently mixed with air is created on one hand, and a part of the injected fuel not sufficiently mixed with air is created on the other hand. In this case, since the air-fuel mixture having an approximately stoichiometric air-fuel ratio, which most easily causes self-ignition, is formed by the part of the injected fuel sufficiently mixed with air, a self-ignition or knocking still occurs. Further, since the remaining part of the injected fuel not sufficiently mixed with air is burned in an atmosphere having a low amount of air, a problem arises in that smoke is generated.
If the injection time is further retarded, self-ignition and knocking do not occur, but another problem arises in that a large amount of smoke is generated.
As mentioned above, in a two-stroke engine, if a large amount of fuel is injected by the fuel injector at one time, when the engine is operating under an intermediate load, various problems arise.